Refactor graph flow notation from "→" to ">" across multiple examples…

[bimantoromaesa]

Refactor graph flow notation from → to > across multiple examples and scripts for consistency. Update expected output structure descriptions to match new notation. Enhance tool registry to require text prompts for zero-shot providers and improve schema generation. Add tests for zero-shot provider detection and schema requirements. Fix minor issues in SAM adapter and video tracking examples.

What

• Replaced → with > in all graph flow examples, scripts, and docs to align with the DSL operator syntax
• Updated output structure descriptions in examples and docs to match the new notation
• ToolRegistry now raises ValueError when text_prompts is missing for zero-shot providers (GroundingDINO, OWL-ViT, CLIP)
• Zero-shot providers auto-include a text_prompts parameter in generated ToolSchema
• Added tests for zero-shot provider detection and text_prompts schema requirement in test_tool_registry.py
• SAM adapter: prompt-less calls now raise a clear error instead of silently returning empty masks
• Video tracking examples: fixed frame iteration and output path handling in examples/track/

Why

Notation inconsistency between → (docs) and > (DSL operator) caused confusion. Zero-shot providers silently producing wrong results without text_prompts was a hard-to-debug footgun. SAM and tracking example fixes address issues found during testing.

Checklist

• Tests pass (pytest tests/ -v)
• Linters pass (black --check src/ tests/ && ruff check src/)
• Type checks pass (mypy src/mata/)
• Documentation updated (if user-facing changes)
• CHANGELOG.md updated (if notable change)